Re: [PATCH v2] mm: do not install PMD mappings when handling a COW fault

[William Kucharski]

> On May 19, 2026, at 02:36, Zhang Yi <yi.zhang@xxxxxxxxxxxxxxx> wrote:
> 
> Gentle ping – could anyone take this patch?
> 
> Thanks,
> Yi.

Could you update the comment to clarify why you shouldn't install PMD mappings
while doing CoW rather than just state it should never be done?

> 
> On 10/24/2025 6:22 PM, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>> 
>> When pinning a page with FOLL_LONGTERM in a CoW VMA and a PMD-aligned
>> (2MB on x86) large folio follow_page_mask() failed to obtain a valid
>> anonymous page, resulting in an infinite loop issue. The specific
>> triggering process is as follows:
>> 
>> 1. User call mmap with a 2MB size in MAP_PRIVATE mode for a file that
>>   has a 2MB large folio installed in the page cache.
>> 
>>   addr = mmap(NULL, 2*1024*1024, PROT_READ, MAP_PRIVATE, file_fd, 0);
>> 
>> 2. The kernel driver pass this mapped address to pin_user_pages_fast()
>>   in FOLL_LONGTERM mode.
>> 
>>   pin_user_pages_fast(addr, 512, FOLL_LONGTERM, pages);
>> 
>>  ->  pin_user_pages_fast()
>>  |   gup_fast_fallback()
>>  |    __gup_longterm_locked()
>>  |     __get_user_pages_locked()
>>  |      __get_user_pages()
>>  |       follow_page_mask()
>>  |        follow_p4d_mask()
>>  |         follow_pud_mask()
>>  |          follow_pmd_mask() //pmd_leaf(pmdval) is true because the
>>  |                            //huge PMD is installed. This is normal
>>  |                            //in the first round, but it shouldn't
>>  |                            //happen in the second round.
>>  |           follow_huge_pmd() //require an anonymous page
>>  |            return -EMLINK;
>>  |   faultin_page()
>>  |    handle_mm_fault()
>>  |     wp_huge_pmd() //remove PMD and fall back to PTE
>>  |     handle_pte_fault()
>>  |      do_pte_missing()
>>  |       do_fault()
>>  |        do_read_fault() //FAULT_FLAG_WRITE is not set
>>  |         finish_fault()
>>  |          do_set_pmd() //install a huge PMD again, this is wrong!!!
>>  |      do_wp_page() //create private anonymous pages
>>  <-    goto retry;
>> 
>> Due to an incorrectly large PMD set in do_read_fault(),
>> follow_pmd_mask() always returns -EMLINK, causing an infinite loop.
>> 
>> David pointed out that we can preallocate a page table and remap the PMD
>> to be mapped by a PTE table in wp_huge_pmd() in the future. But now we
>> can avoid this issue by not installing PMD mappings when handling a COW
>> and unshare fault in do_set_pmd().
>> 
>> Fixes: a7f226604170 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page")
>> Reported-by: Karol Wachowski <karol.wachowski@xxxxxxxxxxxxxxx>
>> Closes: https://lore.kernel.org/linux-ext4/844e5cd4-462e-4b88-b3b5-816465a3b7e3@xxxxxxxxxxxxxxx/
>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
>> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>> mm/memory.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>> 
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 0ba4f6b71847..0748a31367df 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5212,6 +5212,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
>> if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
>> return ret;
>> 
>> + /* We're about to trigger CoW, so never map it through a PMD. */
>> + if (is_cow_mapping(vma->vm_flags) &&
>> +    (vmf->flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)))
>> + return ret;
>> +
>> if (folio_order(folio) != HPAGE_PMD_ORDER)
>> return ret;
>> page = &folio->page;
> 
>